Field of the Invention
The present disclosure relates to an image forming apparatus using an electro photography process, such as a copy machine, a multifunction peripheral, etc.
Description of the Related Art
In some image forming apparatuses of an electro photography type, an electrostatic latent image is formed on a surface of a photosensitive drum by exposing a rotating photosensitive drum with an elongated exposure head (exposure device) having a plurality of light emitting elements. A solid light emitting element, such as an LED (Light Emitting Diode) element or an organic electroluminescence (Electro-Luminescence) element, may be used as a light emitting element. An exposure head includes a plurality of light emitting elements (hereinafter referred to as “light emitting element array”) arranged in a direction of an axis of rotation of a photosensitive drum, and a rod lens array for performing image formation of light from each light emitting element on a photosensitive drum. The exposure head irradiates light on the rotating photosensitive drum. Therefore, on the photosensitive drum, a light spot is formed with its scanning direction being identical to a circumferential direction of the photosensitive drum.
The length of the light emitting element array is defined according to a length of an image formation area of the photosensitive drum in a direction of an axis of rotation. An interval between light emitting elements is defined according to a resolution of the image forming apparatus. For example, when the resolution of the image forming apparatus is 1200 [dpi (dot per inch)], since the pixel interval between the image formed is 21.1 [μm] (rounded to 1 decimal place), the interval between the light emitting element is also set to 21.1 [μm]. The length of the light emitting element array should be longer than that of printing width in the direction of the axis of rotation of the photosensitive drum. The number of light emitting elements is defined by printing width and resolution. For example, when the printing width in the direction of the axis of rotation of the photosensitive drum is 297 [mm], and when the resolution of image forming apparatus is 1200 [dpi], about 14,000 light emitting elements, which is identical to the number of pixels, are arranged in a light emitting element array.
In the image forming apparatus using such an exposure head, as compared to an image forming apparatus of a laser scan type in which a deviation scan with a polygon mirror of a laser beam is performed, since the number of parts used is decreased, it is easy to manufacture the image forming apparatus in a smaller size with more reduced cost. Further, by precisely determining a focal length to the surface of the photosensitive drum to be irradiated, it is possible to decrease the diameter of a light spot formed by exposure of the photosensitive drum. By decreasing the diameter of the light spot, a resolution of an image forming apparatus can be increased, and image sharpness can be increased.
In an exposure head using a rod lens array a focus error may occur on the irradiation surface due to an assembly error of an exposure head, etc. If a focus error occurs, light via each rod lens does not form an image at a point on a surface to be irradiated. In this case, the defective image occurs since the light spot is not formed in the correct shape. In the present disclosure, a light spot which is not formed in the correct shape is referred to “failed spot”. Since the spatial relationship between an emitting element and the rod lens array differs for each light emitting element, when a focus error occurs, an exposure distribution of a failed spot in a scanning direction and that in a direction perpendicular to the scanning direction differ from each other. In order to prevent the defective image by modification due to deformation of a light spot, Japanese Patent Application Laid-open No. 2011-046137 describes an apparatus in which a shape of a light spot is measured and a peak light volume of a light emitting element is adjusted based on the measured result.
In an image forming apparatus which performs a multiple gradation control, when a light spot is deformed to be a failed spot, a concentration variation amount in a low concentration area of an image and that in a high concentration area of the image may differ from each other. For example, since a light spot is large in a low concentration area, there are many isolated dots in the low concentration area, thus the color of the image becomes pale because of the sparseness of the dots. In the high concentration area, since the image tends to be high-density, white color areas are narrowed, thus the color of the image becomes deep. As described in Japanese Patent Application Laid-open No. 2011-046137, since the method of adjusting the peak light volume of a light emitting element cannot adjust concentration for each gradation level, it is not possible to adjust the concentration for all gradation levels. Therefore, an image forming apparatus which prevents the image deterioration resulting from deformation of a shape of a light spot is desired.